Enormous development of GaN based semiconductors has led to the award of Nobel Prize in Physics in 2014. Nevertheless, some fundamental problems of GaN, especially p-T phase diagram, remain unsolved. This is due to severe pressure requirements for stability of this compound at high temperatures.

Ga K-edge EXAFS was used for study of GaN solid-solid phase transition from wurtzite to rock salt structure induced by both pressure at RT and by temperature at constant pressure. Our results allow determining the p-T line describing this structural transition. Experiments were performed at ESRF (Grenoble, France) using nanopolycrystalline DAC as high pressure set-up, KCl as pressure medium and temperature isolation. GaN powder was mixed with Pt micro-powder used as a heating medium. Two Nd:YAG lasers were employed to heat the sample from both sides. The temperature at the opposite edges was monitored by online spectro-radiometry. Pressure was measured with ruby fluorescence method.

The solid-solid phase transition in GaN has been detected at 48 GPa at room temperature under increasing pressure. During decompression a considerable hysteresis (30 GPa) was observed for reverse transition. Obtained values of transition pressures are in good agreement with previous experimental study of this structural transition in GaN by Raman scattering and XANES (PRB 45 (1992) 83). The EXAFS spectra of GaN in rock salt phase up to 70 GPa and its lattice parameters were determined for the first time. After the reverse transition, strongly defected GaN wurtzite lattice still having part of N atoms in positions corresponding to rock salt was observed. This result confirms that N sublattice in GaN is far more stable than the Ga one as predicted by Molecular Dynamics calculations (JAP 96 (2004) 2501).

The temperature induced phase transition was studied at 42 GPa. The wurtzite phase was stable up to 1200 K. At temperature range of 1200 - 1800 K a mixture of both GaN wurtzite and GaN rock salt phases was observed. The solid-solid transition in GaN at 42 GPa was completed at temperature above 1800 K. The experimental observation of the temperature induced solid-solid phase transition in GaN at constant pressure is reported for the first time.

The character of the slope of the studied p-T line is similar to the one previously proposed by Van Vechten in his calculated phase diagram of GaN (PRB 73 (1973)1479). However, the corresponding p-T values are completely different. Therefore, we have performed first principles DFT calculations of temperature dependence of Gibbs potential at several pressures for both crystalline phases of GaN. Obtained results indicated the room temperature solid-solid phase transition at 43 GPa. For temperatures of 926 K and 2770 K the transition pressures of 40 GPa and 35 GPa, respectively, were obtained.  The calculated line is in excellent agreement with our experimental results.

Both experimental and theoretical observation of p-T solid-solid phase transition in GaN is the first step towards complete description of GaN p-T phase diagram. The obtained results help to understand deeper the other fundamental properties (including melting) of this important semiconductor.